1. Field of the Invention
The invention relates to an improved flow conditioner used in tubular conduits carrying single phase fluids. In particular, the invention minimizes metering errors by producing fully developed velocity profile, fully developed turbulence structure, and substantially eliminating swirl of fluids flowing in a conduit.
2. Description of the Related Art
The North American natural gas industry produces, transports and distributes approximately 700 billion cubic meters of gas each year (25 trillion standard cubic feet). The Western European market transports and distributes 250 billion cubic meters of gas each year (9 trillion standard cubic feet). Because of the importance of gas measurement for industry operations and fiscal accountability, it is essential that metering be accurate, reliable, and cost efficient over a range of conditions.
All of this gas is measured at least once, and most of it several times, in meter sizes ranging from 25-900 mm (1-36 inches), at pressures from below atmospheric to 14 MPa (2,000 psi), at temperatures from 0.degree.-100.degree. C. (32.degree.-212.degree. F.), with several types of meters. Large volume metering stations utilize either concentric, square-edged, flange-tapped orifice meters or gas turbine meters.
For over sixty years, the concentric orifice meter has remained the predominant meter of choice for natural gas production, large volume gas flow and chemical metering applications. In fact, it is currently estimated that over 600,000 orifice meters are being used for fiscal measurement applications associated with the petroleum, chemical and gas industries in North America.
All flowmeters are subject to the effects of velocity profile, swirl and turbulence structure of the flowing fluid being measured. Meter calibration factors or empirical discharge coefficients are valid only if geometric and dynamic similarity exists between the metering and calibration conditions or between the metering and empirical data base conditions (i.e., fully developed flow conditions exist). In fluid mechanics, this is commonly referred to as the Law of Similarity.
The classical definition for fully developed turbulent flow is stated by Hinze as follows:
For the fully developed turbulent flow in the pipe the mean-flow conditions are independent of the axial coordinate, x and axisymmetric, assuming a uniform wall condition. PA1 a swirl-free, axisymmetric flow with time average velocity profile and turbulence structure having values approximating those found in fully developed flow and independent of the axial coordinate.
From a practical standpoint, fully developed flow implies the existence of a swirl-free, axisymmetric time average velocity profile in accordance with the Power Law or Law of the Wall prediction. However, fully developed turbulent flow requires equilibrium of forces to maintain the random "cyclic" motions of turbulent flow. This in turn requires that the velocity profile, turbulence intensity, turbulent shear stress, Reynolds stresses, etc., are constant with respect to the axial position.
Unfortunately, fully developed pipe flow is only achievable after considerable effort in a research laboratory. To bridge the gap between research and industrial applications, reference is made to the term pseudo-fully developed flow defined as:
Stated another way, Pseudo-fully developed flow exists when the slope of the orifice meter's discharge coefficient deviation asymptotically approaches zero as the axial distance from the orifice meter to the upstream flow conditioner increases. Of course, this assumes that the empirical discharge coefficient baseline was conducted under fully developed flow conditions.
In the industrial environment, multiple piping configurations are often assembled in series generating complex problems for organizations that write standards and flow metering engineers. The challenge is to minimize the difference between actual or "real" flow conditions in a pipeline and the vertical or research-achievable "fully developed" flow conditions, on a selected metering device's performance to minimize error. One of the standard error minimization methods is to install a flow conditioner in combination with upstream straight lengths of pipe to "isolate" the meter from upstream piping disturbances. Present domestic and international measurement standards provide specifications for upstream straight pipe lengths and flow conditioners upstream of orifice meters. See, e.g., American National Standard Institute (ANSI) (ANSI 2530) and International Standards Organization (ISO) (ISO 5167). Unfortunately, there is considerable disagreement over straight length requirements between ANSI and ISO.
With respect to installation effects and the near term flow field, the correlating parameters which affect similarity vary with meter type and design. However, it is generally accepted that a concentric, square-edged, flange-tapped orifice meter exhibits a high sensitivity to time average velocity profile, turbulence structure, bulk swirl and tap location.
In North America, current design practices utilize short upstream piping lengths with a specific flow conditioner, American Gas Association (A.G.A.) tube bundles, to provide "pseudo-fully developed" flow in accordance with the applicable measurement standard (ANSI 2530/A.G.A. Report No. 3/API (American Petroleum Institute) MPMS Chapter 14.3). Most North American installations consist of 90 degree elbows or complex header configurations upstream of the orifice meter. Tube bundles in combination with piping lengths of seventeen pipe diameters (17*D) have been installed in an effort to eliminate both swirl and distorted velocity profiles. Ten diameters (10*D) of straight pipe is required between the upstream piping fitting and the exit of the tube bundle, and seven diameters (7*D) of straight pipe is required between the exit of the tube bundle and the orifice meter.
In Western Europe, two design practices are currently employed to provide "pseudo-fully developed" flow in accordance with the applicable measurement standard (ISO 5167)--long upstream piping lengths with or without flow conditioners. Most Western European installations consist of complex header configurations upstream of the orifice meter. Piping lengths of one hundred pipe diameters (100*D) without flow conditioners or piping lengths of forty-two pipe diameters (42*D) in combination with flow conditioners have been installed in an effort to eliminate both swirl and distorted velocity profiles.
Three types of flow conditioners have been generally utilized in Western Europe--tube bundles, Zanker and Sprenkle designs. Twenty diameters (20*D) of straight pipe is required between the upstream piping fitting and the flow conditioner, and twenty-two diameters (22*D) of straight pipe is required between the flow conditioner and the orifice meter.
The optimal flow conditioner should achieve a range of design objectives including: a minimal deviation of empirical discharge coefficient (or meter calibration factor) for both long and short pipe lengths; low permanent pressure loss across the flow conditioner (i.e., low "head ratio"); a low fouling rate or a low sensitivity to accumulation of foulants; elimination of swirl; and flexibility for use in both short and long straight lengths of pipe. The latter objective can be achieved by a flow conditioner that produces an axisymmetric, pseudo-fully developed time average velocity profile and turbulence structure. Additionally, it is also desirable that the flow conditioner should be subject to rigorous mechanical design and have a moderate cost of construction.
In the specification and claims, when the swirl angle is less than 2.degree. as conventionally measured by using pitot tube devices, swirl is regarded as substantially eliminated. Further, when the empirical discharge coefficient or meter calibration deviation for both short and long piping lengths is about 1/10 of 1% it is assumed to be at a "minimum".
The ISO and A.G.A. designs, shown in FIGS. 1A and 1B respectively, are intended to eliminate swirl. Both designs include a bundle of tubes having the same length and diameter. For the A.G.A. design (FIG. 1B), the length of the bundle must be at least ten times the tube diameter. For meter runs larger than 75 mm (3 inches) the bundle typically consists of nineteen tubes arranged in a circular pattern with a bundle length of two to about three pipe diameters. For smaller meter runs, the bundle consists of seven tubes arranged in a circular pattern with a bundle length of three pipe diameters. For both the ISO and A.G.A. designs, permanent pressure loss is low, mechanical design is rigorous, cost of construction is low, fouling rate is low, and swirl is eliminated. However, the performance of these devices for minimal deviation from the empirical discharge coefficient for both short and long piping lengths is unacceptable. Also, velocity profile and turbulence structure measurements have shown that both A.G.A. and ISO designs cannot produce pseudo-fully developed flow conditions within reasonable distances due to their high porosity and constant radial resistance. This is shown by the instability in the coefficient performance graphs, FIGS. 19 and 21.
The Sens & Teule flow condition as shown in FIG. 2 is designed to isolate piping disturbances from flow meters. The design consists of a bundle of tubes of different lengths and diameters arranged in a circular array. Permanent pressure loss is high, cost of construction is high, and prototype designs are rigorous and complex. While swirl is eliminated, the fouling rate of this design is unknown. It has been reported that the device exhibits pseudo-fully developed time average velocity profile and turbulence structure for short piping lengths. Geometric scaling of the device is a problem, when considering a range of pipe sizes.
FIGS. 3 and 4, respectively, show the Etoile and Air Moving and Conditioning Association (AMCA) vane-type of swirl eliminator. The Etoile design consists of three flat plates of equal length and width assembled in a star-shaped pattern around a central hub. While these designs eliminate swirl, it is known that the Etoile design does not produce pseudo-fully developed flow conditions in reasonable distances. Similarly, the AMCA design (FIG. 4) was not intended to produce a pseudo-fully developed flow.
FIG. 5 shows an example of screens or wire gauze assembled in an egg-crate fashion within a pipeline. Fine mesh screens are impractical in an industrial environment due to high permanent pressure loss, non-rigorous mechanical construction, and high fouling rates.
Perforated plates, such as the Sprenkle design, shown in FIG. 6, were designed to isolate piping disturbances from flow meters for measuring steam flow. The design consists of three perforated plates spaced one diameter apart and connected by rods. Each plate has a porosity of about fifty percent with regularly distributed perforations in a specified hexagonal pattern. The size of the perforations is about five percent of the pipe diameter. While the designs eliminate swirl, cost of construction is high, design is rigorous and complex, permanent pressure loss is very high, and fouling rate is moderate. Further, performance for minimal coefficient deviation for short piping lengths is unacceptable. Finally, the design is thought to not produce pseudo-fully developed flow conditions for short piping lengths due to its almost constant radial resistance.
The Bellinga design, shown in FIG. 7, is a modified Sprenkle design that suffers much the same shortcomings as Sprenkle.
The Zanker design, shown in FIG. 8, was designed to isolate piping disturbances for the purpose of pump efficiency testing. The device consists of a perforated plate connected to a downstream grid or egg crate construction. The plate includes 32 holes of five different diameters, each hole having a specified location. Permanent pressure loss for this device is high as is cost of construction. While the design eliminates swirl, the design does not provide minimum deviation from empirical discharge coefficient for both short and long piping lengths. Therefore, the design is thought to not produce pseudo-fully developed flow conditions for all piping configurations.
The Akashi design, sometimes referred to as the Mitsubishi design, as shown in FIG. 9, consists of a single perforated plate with 35 holes. The hole size is 13 percent of the pipe diameter and the perforated plate thickness is equal to the hole diameter. The plate has a porosity of approximately 59 percent. Hole distribution is dense toward the (center of the pipe) and sparse around the periphery (pipe wall). The upstream inlets of the holes are beveled. While the device produces a low permanent pressure loss and mechanical design is rigorous and simple, performance for minimal deviation from empirical discharge coefficient for both and short and long piping lengths are unacceptable since the design calls for almost constant radial resistance. The design is further thought to not produce pseudo-fully developed flow conditions for all piping configurations because it does not provide minimal deviation from the empirical discharge coefficient for both short and long pipe lengths.
The Laws device, shown in FIG. 10, is also a single perforated plate, but with 21 holes. The plate thickness is approximately 12 percent of the pipe diameter (D) and the plate has a porosity of about 51 percent. The holes are arranged in circular spaced arrays around a central hole. The first and second arrays have 7 and 13 holes respectively. Hole size is largest in the middle of the pipe, 0.1924*D, and decreases in size to the first circular array, 0.1693*D, and further in size to the second array, 0.1462*D. The pitch circle diameter for the first and second array are about 46 and 84 percent of the pipe diameter respectively. Upstream inlets of the holes may be beveled. Once again, the performance for minimal deviation from the empirical discharge coefficient for short piping lengths is unacceptable, but acceptable for long piping lengths. The design can produce axisymmetric, pseudo-fully developed conditions only for long piping lengths.
A further development of a device shown in U.S. Pat. No. 5,255,716 to Wilcox is the K-Labs Mark V. The patent shows a flow conditioner comprising tubular passages with the area between specific tubes blocked. While permanent pressure loss is low and mechanical design is rigorous and simple, and swirl is eliminated from most piping configurations, the performance for minimal deviation from the empirical discharge coefficient for short piping lengths is unacceptable. Therefore, the design is thought to not produce pseudo-fully developed flow conditions for all piping configurations.
What is yet required is a flow conditioner for use with flow meters to provide measurements that are sufficiently accurate for industrial and fiscal applications. The flow conditioner should achieve all the design criteria stated above including elimination of swirl, and achievement of a minimal deviation from the empirical discharge coefficient or meter calibration factor for both short and long straight lengths of pipes by the production of a pseudo-fully developed time average velocity profile and turbulence structure. Further, the device should have a low permanent pressure loss (head ratio) across the flow conditioner, low fouling rate or insensitivity to foulant accumulation. Finally, the device should be subject to a rigorous mechanical design and should have a relatively moderate cost of construction.